Personally I dont think there should be a maximum age limit as to how old you can be to join or serve in the armed forces.
Certainly there is a job somewhere that even a 70 year old can do.
Brings to mind the story of John L burns

70 year old veteran of 1812 volunteered to fight at the battle of Gettysburg as a sharpshooter. Fought with with the 7th Wisconsin Infantry and the 24th Michigan "Iron Brigade"
After managing several kills. Including one of shooting a charging confederate officer off his horse, he was wounded 3 times arm. Leg and smaller wounds to the chest.
Takes some balls. Having been through one war 40 years earlier and knowing full well what it is like. To be wiling to get it up one more time and go through it again.
In any event. I think it should be every citizens right to serve in the military regardless of age.(note I said IN, not for)
In the private sector its called age discrimination